Robert M Watson Jr
I do not recall the earliest reference to the jewel (Yayoi period) but I do seem to recall the shape you refer to has archeological sources that predate the kojiki by ~1700 years and they do appear, more or less, in the 'standard' shapes.
Take two 'standard' jewels and place them together and, viola, round in a blend of in yo (yin-yang aka 'diagram of ultimate power'). Six of them suckers in complimentary pairs in three orthogonal planes gives a sphere ... sometimes a cigar is just a cigar.
Sometimes - but I have to consider that Ueshiba Loved
(with a capital L) symbolism and multiple layers of meaning. Also, the artist was certainly instructed to portray certain things in a certain way.
Still, it's just my speculation, since I haven't seen mention of it elsewhere (that doesn't mean that it hasn't been mentioned somewhere, just that I haven't seen it or don't remember).